Anatomy

• nostrils
• trachea
• bronchi
• lungs: bronchi that bring air to lungs divided into many smaller tubes; lung made up of many tiny tubes (unlike mammals, whose lungs are made of tiny, dead-end sacs) that run through the lung, and through which air passes.  Tubes contact capillaries for gas exchange.
• air sacs: sacs attached to lungs, bronchi, into which air passes during parts of breathing.  In neck, front of body, upper chest, abdomen; some extend into bones, syrinx.  Function in allowing efficient air-flow through lungs, help remove heat produced by high metabolism during flight, cushion internal organs during flight, sound production.

• inspiration (breathing in): air passes into air sacs
• expiration (breathing out): air passes through lungs, and out
• the breathing cycle: movement of air through lungs (see Figure 1)
• result: efficient, one-way air flow through lungs; almost no dead air space in lungs

Figure 1. Movement of a "slug" of air through the respiratory system of a bird